As is well known in the air conditioning art, and more particularly those employing hermetic refrigeration systems, maximum efficiency of an evaporator is attained by maintaining the refrigerant stream leaving the evaporator in a saturated gaseous state so that the entire heat transfer surface of the evaporator is subjected to heat absorption by vaporization. With this ideal condition, the refrigerant absorbs latent heat in the evaporator and no sensible heat to raise its temperature following vaporization with the result that the maximum available refrigerating effect is attained. It has been general practice in the refrigeration industry to size evaporator coils with an amount of surface and pressure drop to assure that the refrigerant leaving the evaporator is in an expanded and superheated gaseous state.
The condenser, on the other hand, is designed to provide totally liquid phase refrigerant to the expansion or capillary valve, which as is well known cannot tolerate any significant amount of refrigerant gas. Consequently, the refrigerant must be totally condensed to a liquid phase in the condenser.
Conventional heat pump refrigeration systems of the type to which this invention particularly relates comprise indoor and outdoor coils or heat exchangers connected to a closed refrigerant circuit. Refrigerant is circulated through the coils by a compressor which pumps the compressed refrigerant gas through the coil where it is condensed and passes through a means for expansion, such as a capillary tube or expansion valve, to the other coil for evaporation. The system includes suitable change-over valve mechanisms for reversing the function of the indoor and outdoor heat exchangers permitting the indoor exchanger to function as an evaporator for summertime cooling or as a condenser for wintertime heating, the other coil performing the opposite function.
One of the shortcomings of the prior art heat pump refrigeration systems of the type described above is their incapability of the heat exchangers to operate efficiently both as evaporators and condensers. This is especially true since it takes a greater pressure drop through the condenser to change the high pressure gas to a high pressure liquid than it does for the evaporator to change low pressure liquid to a low pressure gas. Accordingly, in heat pump or reverse cycle refrigeration systems when the coils designed to operate as evaporators and condensers are reversed in the refrigeration cycle, they are inefficient.
In other prior cooling systems an auxiliary coil has been used to increase the subcooling of the condensed refrigerant, usually in conjunction with a liquid receiver. In this arrangement all of the condensing coil can be used to condense high pressure gas to a liquid. The receiver collects the extra liquid so it does not back up into the condenser using up condensing surface. The liquid then feeds from the receiver to the specialized subcooling coil where it is further cooled to provide added capacity to the system. This system does not function well in reverse, as an evaporator, because of the excessive pressure drop of evaporating refrigerant passing through the subcooling coil.
In still other prior art arrangements such as 3,024,619- Gerteis, for application in a heat pump the auxiliary coil is alternately connected to the main heat exchanger as a subcooling coil when the heat exchanger is condensing and integrated as part of the evaporator when the heat exchanger functions as an evaporator.